A. Field of the Invention
This invention relates to a method of hydraulically fracturing a subterranean formation, particularly a hydrocarbon bearing formation though equally applicable to water or steam bearing formations, penetrated by a borehole, and more particularly relates to a method of hydraulic fracturing wherein fluids are injected in a series of stages to create multiple fractures.
B. Description of the Prior Art
The art of hydraulic fracturing of subterranean formations is well known.
Various techniques have been proposed for placing propping agents in fractures to prevent the fractures from completely closing, or "healing", when the wellhead pressure is relieved. Most involve the injection of multiple stages of fluids. Henry, U.S. Pat. No. 3,245,470 employed alternating foam stages to achieve deposition of proppant. Braunlich, Jr., U.S. Pat. No. 3,335,797 teaches a method for controlling the downward growth of fractures by a prop placement technique. Hanson et al., U.S. Pat. No. 3,151,678, teach to impart a surging action to the proppant as it is injected. Tinsley in U.S. Pat. Nos. 3,592,266 and 3,850,247 teaches methods whereby an effort is made to prop the fracture at intermittently spaced intervals.
Kiel, U.S. Pat. No. 3,933,205, and Winston, U.S. Pat. No. 3,948,325, teach methods of fracturing for creating multiple fractures, wherein the formation is permitted to heal at least partially between injection stages. In Kiel, the intermediate healing step is said to create spalling of the fracture faces. In Winston, the relaxation step following injection of what the patentee calls a "Bingham plastic fluid" is said to create a long plug against which a pressure can be applied to create a second fracture. In both Kiel and Winston, the high viscosity fluid may carry a proppant. Where Kiel employs a proppant, he teaches to follow the proppant stage with a viscous flush, e.g. Super Emulsifrac fluid having no proppant, prior to the healing step. See, for example, the treatment report in columns 21 and 22, Event Nos. 8-10. Winston teaches the Bingham plastic fluid may contain a propping agent (col. 4, line 22,) and may be followed by displacement fluid (col. 3, lines 32-34). In Example 1, Winston follows a borate gelled guar fluid containing proppant with a water stage prior to relieving pressure. In neither Kiel nor Winston, however, is it taught to follow the proppant stage with both a viscous, proppant free spacer and a non-viscous proppant free fluid, prior to the relaxation step.